I am running a 5.0 roller motor with E303 cam in a 66 Mustang. I went with a set of stock 1969 351w heads. My problem is at high RPM's, 5000 - 6000, I calapsed 2 of the lifters. This is what the shop told me anyway. I had the lifters replaced and the engine is very reliable provided I keep it below 5000rpm. Does anyone know why this combination would not work? Could the 351w springs be too stiff for a roller motor? Other engine components are Edelbrock performer intake, Edelbrock 600cfm carb, toploader 4spd, 3.50 posi gears.